Automatic Battery Float Charger

This lightweight 12v battery charger lets you skip the hassle of recharging a battery by maintaining the charge in your car battery. The automatic battery float charger features an automatic safety shut off and copper battery clamps for better conduction. Lightweight and easily stored for quick access, this 12v battery charger is a must-have safety precaution for battery maintenance while traveling.

These chargers are FLOAT chargers. They are NOT designed to charge batteries from a discharged state. (These chargers are capable of only 1/2 amp at 14+ volts.) They are designed to...Read complete review

These chargers are FLOAT chargers. They are NOT designed to charge batteries from a discharged state. (These chargers are capable of only 1/2 amp at 14+ volts.) They are designed to maintain a fully CHARGED battery in its fully charged or current state, not charge a discharged battery. They do this by monitoring the battery voltage via the circuitry in the little square box in the charge line, and providing a charging voltage just a little bit higher, 0.2 to 0.6 volts, than the at-rest voltage. A fully charged lead-acid battery has a voltage around 12.9 volts, and is "fully discharged" at around 11.4 volts. (These values are based on the specific gravity of the battery acid and vary depending on the chemistry of the battery, i.e, flooded, gel, glass mat, sealed, etc.) Below 11 volts, this charger should not do anything other than possibly discharge the battery. Reading all the comments, the chargers worked properly. They did the job they're supposed to, MAINTAIN the charge in a battery, although they could probably use some circuity to prevent totally discharging the battery. One user actually turned the float charger into a regular charger by inserting a diode into one of the lines. This effecively disables the circuitry in the little control box and allows the charger to apply its full voltage, around 14+ to the ends of the cables instead of the maximum 13.2 volts when the control circuit is working. The diode prevents it from discharging the battery, but also disables the "float" feature, and can overcharge the battery. Basically, the chargers were working as advertised.

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Most Liked Negative Review

OK for slow charging, not float

I am an electrical engineer and have the equipment necessary to fully test this charger.

There is nothing "automatic" about this charger, and it violates its own spec (which...Read complete review

I am an electrical engineer and have the equipment necessary to fully test this charger.

There is nothing "automatic" about this charger, and it violates its own spec (which says maximum output is 13.2 V, when it is actually 14 V). Its output voltage is too high to be considered a continuous float charger, since the battery could be overcharged and, eventually, the electrolyte fluid boiled out leaving it dry and dead. I saw no evidence of it trying to regulate its output voltage to 13.2V using either a real (well-charged) car battery or resistive loads. Others have also mentioned that the voltage is too high to be a float charger and leave on all the time, I agree.

HOWEVER, this charger could be used as an inexpensive (but slow, low-current) battery charger. We could call it a 0.5 amp charger since that is its initial current into a battery that is not too discharged. Roughly speaking, at 0.5 A it would take at least 60 hours to put 60 ampere-hours into the battery, so if the battery has reserve capacity of 75 minutes or longer, which translates into a little over 30 ampere-hours (for a 25 A reserve load), it would take roughly 60 hours or 2.5 days to charge a dead battery using this charger (actually a bit longer due to charging inefficiency). If the battery is only "partially dead" it may take less time, maybe only a day or two. Of course if the battery has been sitting dead very long it may not accept a charge anymore no matter how long you charge it, in which case you will need to get a new battery.

This charger has a regulated and filtered output so even under load the peak voltage never exceeds 14V, so it is safe to apply even if the battery has developed a very high internal resistance and behaves as almost an open circuit, without having to disconnect the battery (this voltage will not harm connected always-on electronics such as clocks). Be sure all electronics are turned off because this charger has barely enough current to charge the battery (not enough to supply power to electronics that are turned on).

For battery maintenance on a car or motorcycle battery that is not in use, it could safely be applied overnight every couple weeks, but do not leave it on more than about a day (unless you know the battery was nearly dead to begin with, then maybe up to 2.5 days). If you leave it on *all* the time you will slowly evaporate the electrolyte until the battery is ruined.

When connecting the charger, connect the red lead to "+" and black lead to "-" battery terminals, then plug it into AC. To disconnect, unplug from AC, then disconnect one or both battery leads. DO NOT LEAVE THE LEADS CONNECTED TO THE BATTERY VERY LONG IF THE CHARGER IS UNPLUGGED because it draws 31 mA to power its LED and internal electronics, which would eventually discharge the battery.

I would only recommend this charger to someone who wants a low-cost charger and does not mind leaving it on over night (because it is very slow), but will remember to disconnect it after 12-24 hours so that it does not overcharge (and ruin) the battery. I would NOT recommend it to someone who wants to leave it on the battery all the time.

MORE TECHNICAL DETAIL FOR THOSE INTERESTED:

For various load currents I measured the charger's output voltage as follows:

For various load currents I measured the charger's output voltage as follows (sorry I had to use underscores to align the columns since the website filters out tabs or multiple spaces):

I applied this to my own car battery (which was in good shape and fully charged) for about 12 hours as a test. Prior to charging the battery voltage was 12.64 V. Measured the following voltages across battery and charging currents over 12.5 hour period, with ambient temperature in garage about 55 deg F:

These have saved me from purchasing numerous batteries over the last 20 years, having extended the life of batteries years past normal life. I keep them on all outdoor products that are stored more than they get used. I keep one on each machine for the off season storeage of each machine too. Also periodically connect to spare stored batteries a week at a time per battery. These float chargers also keep vehicles being stored from slowly discharging from electronics that draw small current while not being driven. I keep them connected continually when not in use; never have overcharged a battery.

I have equipment and vehicles that go unused for months. I have one of the float chargers attached to all of these and it keeps the battery ready to go. I've used these for several years, never had trouble with one. The battery that it's hooked to has always been ready to go when I need it.

I use the chargers for my four riding mowers, tractors, generator and extra car. Since using them, I've not had a problem with a dead batter. They connect/disconnect easily and can be stored in smaller spaces. I keep and extra charger (still in original package) hanging in each shed for unexpected guest vehicles. They are nice to use as gift.

I have had this little (cheap) battery tender on my tractor battery for about a year. My tractor will sit unsed for months at a time. But when I need it, it cranks right up with a strong battery. REMEMBER, this is not a charger. fully charge your battery before implementing this little life (battery) saver. Also, it does NOT work for a gel battery. So if you have a motorcycle gel battery, you will have to spend a little more on a slightly more advanced tender. You can't beat this thing for the price.

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